In trucks of the type having a tractor portion and a trailer portion, the trailer portion is normally provided with air brakes. The pedal for applying the trailer brakes and releasing them is conventionally located in the cab of the tractor portion. Upon release of the brake pedal, the supply or tractor portion of the brake line is exhausted through a valve located under the cab. It is known to provide a valve in the air brake line between the pedal and the trailer brakes to vent the trailer portion of the line upon release of the brake pedal. This helps eliminate or reduce lag time between release of the brake pedal and release of the trailer brakes. A known type of vent valve has opposite coaxial inlet and exhaust ports and a perpendicular delivery port. A flexible diaphragm is positioned between the inlet and exhaust ports. The diaphragm deflects to close communication with the exhaust port when the brakes are applied and to close communication with the inlet port when the brakes are released. In each case, the diaphragm seals against a valve seat to seal off all communication with the respective port.
A problem that has arisen in connection with this known type of valve is that the diaphragm tends to reseal against the exhaust seat before the trailer end of the brake line is completely exhausted. This creates a residual positive pressure in the trailer end of the brake line and a residual pressure differential across the diaphragm. The residual positive pressure in the trailer brake line can result in dragging of the trailer brakes and consequent excessive wear of the trailer brakes. The dragging of the brakes also leads to excessive wear of the trailer tires. Another problem associated with the conventional type of valve is that the inlet and delivery ports are not coaxial. Therefore, the valve is not line mountable and does not provide a direct flow pathway between the tractor and trailer portions of the brake line.
Valves of the type described above are disclosed in K. A. Brandenberg U.S. Pat. No. 3,196,890, granted Jul. 27, 1965; P. Beaumont U.S. Pat. No. 4,461,313, granted Jul. 24, 1984; D. J. Goodell U.S. Pat. No. 4,596,265granted Jun. 24, 1986. J. Goodell. The Beaumont and Goodell valves are specifically described as being for vehicle braking systems. The Beaumont valve is designed to minimize or eliminate the hiss which accompanies the exhausting of "conventional" valves. The diaphragm in the Beaumont valve has a central working area and a peripheral support area. Indents or cutouts are formed in the peripheral area to minimize noise produced by vibration of the diaphragm during exhausting of the delivery portion of the line.
R. R. Crookston U.S. Pat. No. 2,716,997, granted Sept. 6, 1955, discloses a quick release valve for pressurized air supply lines connected to devices such as pumps and engine clutches of industrial engines. The valve has coaxial inlet and delivery ports and is coaxial with the supply line. It is designed to supply air pressure at a controlled rate and allow rapid release of pressure. A sliding valve member has a delivery position in which it seals a radially directed exhaust port. When a remote valve is operated to cut off the supply of pressurized air and bleed the supply line, a pressure differential is created and moves the valve member to an exhaust position. In one of the embodiments disclosed by Crookston, the valve inlet port is not completely sealed from the delivery port when the valve member is in the exhaust position. A slidable element within the valve member has a small central axial orifice that allows a small amount of flow past the valve element. The described purpose for this arrangement is that the valve position is not changed if an operator opens or closes the remote valve and quickly changes his mind. In other words, it creates an intentional lag in the response of the valve element in moving to either its delivery position or its exhaust position.
The above-cited patents and the prior art that is discussed and/or cited therein should be studied for the purpose of putting the present invention into proper perspective relative to the prior art.